Pharmacokinetics

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    Pharmacokinetics describes how the body affects a specific xenobiotic/chemical after administration through the mechanisms of absorption and distribution, as well as the metabolic changes of the substance in the body (e.g. by metabolic enzymes such as cytochrome P450 or glucuronosyltransferase enzymes), and the effects and routes of excretion of the metabolites of the drug.[1] Pharmacokinetic properties of chemicals are affected by the route of administration and the dose of administered drug. These may affect the absorption rate.[2]

    Metrics[edit | edit source]

    The following are the most commonly measured pharmacokinetic metrics:[3] The units of the dose in the table are expressed in moles (mol) and molar (M). To express the metrics of the table in units of mass, instead of Amount of substance, simply replace 'mol' with 'g' and 'M' with 'g/dm3'. Similarly, other units in the table may be expressed in units of an equivalent dimension by scaling.[4]

    Pharmacokinetic metrics
    Characteristic Description Symbol Unit Formula Worked example
    value
    Dose Amount of drug administered. N/A (Design parameter) 500 mmol
    Dosing interval Time interval between drug dose administrations. N/A (Design parameter) 24 h
    Maximum serum concentration The peak plasma concentration of a drug after administration. N/A (Direct measurement) 60.9 mmol/L
    Minimum time for Cmax Minimum time taken to reach Cmax. N/A (Direct measurement) 3.9 h
    Minimum plasma concentration The lowest (trough) concentration that a drug reaches before the next dose is administered. 27.7 mmol/L
    Average plasma concentration The average plasma concentration of a drug over the dosing interval in steady state. 55.0 h×mmol/L
    Volume of distribution The apparent volume in which a drug is distributed (i.e., the parameter relating drug concentration in plasma to drug amount in the body). 6.0 L
    Concentration Amount of drug in a given volume of plasma. 83.3 mmol/L
    Absorption half-life The time required for 50% of a given dose of drug to be absorbed into the systemic circulation.[5] 1.0 h
    Absorption rate constant The rate at which a drug enters into the body for oral and other extravascular routes. 0.693 h−1
    Elimination half-‍life The time required for the concentration of the drug to reach half of its original value. 12 h
    Elimination rate constant The rate at which a drug is removed from the body. 0.0578 h−1
    Infusion rate Rate of infusion required to balance elimination. 50 mmol/h
    Area under the curve The integral of the concentration-time curve (after a single dose or in steady state). 1,320 h×mmol/L
    Clearance The volume of plasma cleared of the drug per unit time. 0.38 L/h
    Bioavailability The systemically available fraction of a drug. Unitless 0.8
    Fluctuation Peak–trough fluctuation within one dosing interval at steady state.

    where

    41.8%

    See Also[edit | edit source]

    References[edit | edit source]

    1. Pharmacokinetics. (2006). In Mosby's Dictionary of Medicine, Nursing & Health Professions. Philadelphia, PA: Elsevier Health Sciences. Retrieved December 11, 2008, from http://www.credoreference.com/entry/6686418
    2. Knights et al.; "Pharmacology for Health Professionals" , ISBN: 0-7295-3664-5
    3. Collection of terms, symbols, equations, and explanations of common pharmacokinetic and pharmacodynamic parameters and some statistical functions, http://www.agah.eu/fileadmin/_migrated/content_uploads/PK-glossary_PK_working_group_2004.pdf
    4. "Half Life" , StatPearls , StatPearls Publishing , http://www.ncbi.nlm.nih.gov/books/NBK554498/
    5. "Half Life" , StatPearls , StatPearls Publishing , http://www.ncbi.nlm.nih.gov/books/NBK554498/